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We studied the spreading of phospholipid bilayer membranes and the conditions for the formation of continuous bilayers on rough (glass, glass-MgF2, glass-MgF2-SiC> 2) and smooth (mica) solids using reflection interference contrast microscopy as an analytical tool. We show that two fundamentally different spreading mechanisms are possible:(i) The sliding of a single bilayer on a thin lubricating water film and (ii) the rolling of thin lobes of two juxtaposed bilayers in a tank tread type motion. In the first mechanism the spreading velocity of a straight interface exhibits a square root behavior, v~ f~ y2, allowing an estimate of the frictional coupling of the membrane to the substrate. On smooth surfaces (eg, freshly cleaved mica) the dissipation is dominated by shear flow in the ultrathin water film separating the bilayer from the substrate. On rough surfaces in contrast (eg, glass) friction is caused by two-dimensional flow of …